Internal support for superconductor windings

ABSTRACT

The invention features an internally supported superconducting coil assembly. The invention includes several superconducting windings and at least one internal coil support member that forms a laminate stack alternating between an internal support member and a superconducting winding.

This invention arose in part out of research pursuant to Contract No.N00014-99-C-0296 awarded by the Office of Naval Research.

TECHNICAL FIELD

This invention relates to the construction and operation ofsuperconducting rotating machines, and more particularly tosuperconductor winding construction for use in superconducting motors.

BACKGROUND

Superconducting air core, synchronous electric machines have been underdevelopment since the early 1960s. The use of superconducting windingsin these machines has resulted in a significant increase in the magnetomotive forces generated by the windings and increased flux densities inthe machines. However, superconducting windings generate tremendousinternal stresses that attempt to force the superconducting windingsinto circular shapes. Certain applications require the superconductingwindings to be non-circular for various reasons and the internalstresses must be alleviated or supported.

SUMMARY

The invention features an internally supported superconducting coilassembly. The invention includes several superconducting windings and atleast one internal coil support member that forms a laminate stackalternating between an internal support member and a superconductingwinding. Embodiments of this aspect of the invention may include one ormore of the following features.

The internal coil support members are especially advantageous whennon-circular superconducting windings are utilized. In certainembodiments, a racetrack shaped superconducting winding is used. Theracetrack shape is defined by two opposing arcuate end sections and twosubstantially straight side sections. The internal magnetic stressesgenerated by the superconducting winding attempts to force thesuperconducting winding to become round in shape. The internal coilsupport members help alleviate the internal stresses. The internal coilsupport members work better than external support members because thebending stresses are greatest near the center of the winding, away fromany external supports.

In certain embodiments, the superconducting coil assembly laminate canbe fixed to a rotor body for use in a rotating machine by passing a boltthrough the laminate and into the rotor body. The bolt, or multiplebolts, will help unify the laminate into a unitary whole. The laminatemay also be impregnated with epoxy to achieve a unitary whole.

The internal coil support members must have openings to allow electricalconnection between adjacent superconducting windings that are separatedby the internal coil support member. The internal coil support member isusually made of stainless steel, which further helps quench the magneticforces.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional perspective view of a superconducting motorin accordance with the invention.

FIG. 2 is a generic cross-sectional view of the superconducting motor ofFIG. 1.

FIG. 3 is a perspective view of a stator assembly of the superconductingmotor of FIG. 1.

FIG. 4 is a perspective view of a single phase of stator coils of thestator assembly of FIG. 3.

FIG. 5 is a perspective view of a single phase of stator coils mountedon the support tube of the stator assembly of FIG. 3.

FIG. 6 is a cross-sectional perspective view of a stator coil section ofthe stator assembly of FIG. 3.

FIG. 6A is a schematic of two stator coils and an associated coolingloop.

FIG. 7 is a cross-sectional perspective view of a rotor assembly of thesuperconducting motor of FIG. 1.

FIG. 8 is a cross-sectional perspective view of an output shaft andvacuum chamber of the rotor assembly of FIG. 7.

FIG. 9 is a perspective view of rotor coils mounted on a rotor body ofthe rotor assembly of FIG. 7.

FIG. 10 is a cross-sectional view of the rotor coil stack with internalsupport members of the rotor coils of FIG. 9.

FIG. 11 is a perspective view of an axial buckle of the rotor assemblyof FIG. 7.

FIG. 12A is a perspective view of a tangential buckle of the rotorassembly of FIG. 7.

FIG. 12B is a perspective view of the tangential buckle of FIG. 12mounted with a spring.

FIG. 13A is a cross-sectional perspective view of the tangential bucklesmounted within the rotor assembly of FIG. 7.

FIG. 13B is a cross-sectional perspective view of the axial bucklesmounted within the rotor assembly of FIG. 7.

FIG. 14 is a perspective view of a cryogenic cooling system and mountingflange of the superconducting motor of FIG. 1.

FIG. 15 is a block diagram of a cryogenic cooling system of thesuperconducting motor of FIG. 1.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1 and 2, a superconducting synchronous motor 10includes a rotor assembly 50 cooled by a cryogenic cooling system 100,here a Gifford McMahon (GM) cooling system, and surrounded by a statorassembly 20. Both the stator assembly 20 and the rotor assembly 50 aremounted in a housing 12 to protect the components and any users of thesuperconducting motor 10. As will be described in greater detail below,each of these components and assemblies have features which contributetoward both increasing the overall performance, as well as reducing theoverall size of motor 10. In particular, superconducting synchronousmotor 10 can be shown to produce torque densities as high as 150 N·m/Kgor more at 300 RPM or less. Furthermore, such motors are expected toprovide a greatly improved gap shear stress characteristic in a rangebetween 30 psi and 100 psi.

Referring to FIGS. 1 and 3-5, the stator assembly 20 includes, in thisembodiment, one hundred eight stator coils 22 wound around a supporttube 34, and arranged in a multi-phase configuration, here a 9-phaseconfiguration. The twelve stator coils 22 per phase provide a 12-polearrangement. A back iron 36 is constructed by wrapping magnetic wirearound the stator coils 22. The stator coils 22 are wound into a diamondpattern, with one stator coil 22 diamond representing a single pole. Thestator coils 22 are arranged around the support tube 34 by overlappingsides of adjoining stator coils 22 in the same phase.

Referring to FIG. 6, cooling conduits 30 are positioned to be in thermalcontact with each stator coil 22 to facilitate cooling of the statorassembly 20. Each cooling conduit 30 is constructed from a thin walled,high electrical resistivity alloy for minimizing eddy current heating.Each coolant passage of the cooling conduit 30 is distinct andelectrically isolated from the adjacent coolant passage. Because thecooling conduits 30 are generally constructed from an electricallyconductive material, an electrically insulating tape 28 is wrapped aboutthe stator coil 22 to electrically insulate the stator coil 22 fromsurrounding components that are at ground potential, particularly thecooling conduits 30. In particular, the electrically insulating tape 28maintains the cooling conduits 30 at ground potential, therebypermitting the use of fresh water, which contains ions. The electricallyinsulating tape 28 is made from a material having a thickness that canwithstand operating voltages of the conductor turns 24, as well as theheat generated by the conductor turns 24. The thickness of theelectrically insulating tape 28 is determined by the dielectric strength(insulating properties) of the material and operating voltage, typicallybetween about 0.001 to 0.100 inches. Examples of materials for theelectrically insulating tape 28 include, but are not limited to, epoxy,mica, and glass tapes.

In this embodiment, the stator coils 22 are formed of an array ofmultiple conductor turns 24. Each conductor turn 24 is electricallyisolated from an adjacent turn by insulation 26. Insulation 26 may beformed of the same material as electrically insulating tape 28, but hasa reduced thickness (e.g., 0.001 to 0.030 inches).

Referring to FIGS. 6 and 6A, cooling conduits 30 are mounted adjacent toand in contact with the electrically insulating tape 28 surrounding eachstator coil 22. Each cooling conduit 30 has a number of passagesextending therethrough for receiving a coolant from a fresh waterexternal source 200. With reference to FIG. 3, each cooling conduit 30has an opening (not shown) at the end regions of each stator coil 22.Therefore, one hundred eight openings are in fluid communication with amanifold assembly (not shown) to allow fluid into each cooling conduit30 from the external source 200. On the other side of the stator coils22, one hundred eight openings are in fluid communication with a return202. In one embodiment, the manifolds are end caps (not shown)circumferentially mounted to the front and back edge of the statorassembly 20.

A porous copper thermally conductive member 32, which has low electricalconductivity, is disposed about the stator coil 22 and cooling conduits30 to facilitate cooling of the entire stator coil 22. In otherembodiments, this could be constructed from a wire disposed about thestator coil 22. Absent the thermally conductive member 32, the statorcoil 22 would only be cooled at the contact point between the coolingconduit 30 and the electrically insulating tape 28. Because of thiscontact point cooling, a thermal gradient would be induced through theelectrically insulating material 28. This thermal gradient createsthermal stresses between the cooling conduit 30 and the electricallyinsulating tape 28, which can cause premature failure in the statorassembly 20 due to electrical breakdown at this interface. Additionally,with high power density embodiments, the cooling conduit 30 cannot bemounted on a wide side of the stator coil 22 due to the required highpacking densities. To minimize the peak temperature, the thermallyconductive member 32 is positioned around the stator coil 22 and thecooling conduit 30 to allow heat transfer from the sides of the statorcoil 22 that are not in direct contact with the cooling conduit 30.

In certain embodiments, cooling of the stator assembly 20 is furtherenhanced by varying the thickness of the electrically insulatingmaterial 28. The electrically insulating material 28 isolating theconductor turns 24 in each diamond-shaped stator coil 22 from thegrounded thermally conductive member 32 experiences varying dielectricstress dependent on the electrical location of the coil within a givenphase of the stator assembly 20 with stator coils 22 connected inseries. The two stator coils 22 at the end of the phase are connecteddirectly to line voltage and their electrically insulating material 28experiences maximum dielectric stress between conductor turn 24 and thethermally conducting member 32. The coils electrically located midwaybetween the ends of the phase are exposed to approximately half thedielectric stress due to the voltage drops in the stator coils 22between the end and middle of the phase. The thickness of theelectrically insulating material 28 is varied in uniform steps directlyproportional to the voltage variation. In one embodiment, the minimumthickness of the electrically insulating material 28 thickness iscalculated by the relationship T_(ins)*(0.5+(1/N)), where T_(ins)represents the maximum thickness of the electrically material 28 atcoils connected to the line voltage and N represents the even number ofstator coils 22 in each phase. The electrically insulating material 28thickness will proportionally vary in uniform steps between the maximumthickness, T_(ins), and the minimum thickness. Varying the thickness ofthe electrically insulating material 28 will help facilitate cooling,since thicker electrically insulating material 28 will not be used whereit is not needed.

In another embodiment, the stator coils 22 in each phase may be arrangedand connected in pairs in a two layer winding with stator coils 22having the thinnest and thickest electrically insulating material 28being paired. Stator coils 22 with the next thinnest and next thickestelectrically insulating material 28 are then paired, this process beingcontinued until the final two middle stator coils 22 are paired.

In certain other embodiments, the benefits of varying the thickness ofthe electrically insulating material 28 can be enhanced by varying thecross sectional area of each of the two stator coils 22 in the abovedescribed pairs of stator coils 22. The cross sectional area of theconducting turns 24 in the stator coil 22 with thin electricallyinsulating material can be decreased as higher power can be dissipateddue to the decreased thermal resistance of the thin electricallyinsulating material 28. This makes room in the same coil pair todecrease the power dissipation in the remaining coil with thickelectrically insulating material 28 by increasing the cross sectionalarea of its conducting turns 24. Typically winding temperature rise isreduced by 30 percent compared with the result of using conventional artwith uniform insulation thickness and uniform wire cross sectionalareas. Increased resistance to voltage breakdown between the conductingturns 24 and the adjacent thermally conductive member 32 can be obtainedcompared with conventional art by increasing the thickness ofelectrically insulating material 28 on each of the coils in the abovecoil pairs for the same higher temperature as obtained with conventionalart.

Referring to FIG. 7, the rotor assembly 50 includes a rotor body 58,onto which the superconducting rotor coils 52 are fixed, mounted onto anoutput shaft 82 by an array of tangential buckles 70 and axial buckles60. As will be explained in detail below, the tangential buckles 70 andthe axial buckles 60 transfer the torque and forces produced by therotor coils 52 to the output shaft 82, while also thermally isolatingthe cryogenically cooled rotor body 58 from the output shaft 82. Thetangential buckles 70 and axial buckles 60 are mounted between rotorbody ribs 59 and output shaft plates 84, as will be described in detailbelow. Vacuum chamber walls 86 are integrally mounted to the outputshaft 82, enclosing the rotor assembly 50 and acting as a cryostat. Aswill be described in detail below, a closed cryogenic cooling loop 118(Shown in FIG. 2) is used to conduct heat from the rotor coils 52 to thecryocooler 104 where the heat can be dissipated. In particularembodiments, vacuum chamber 86 includes an outer cylindrical wall that,for reasons discussed below, serves as an electromagnetic shield 88.

Referring to FIGS. 7 and 8, the output shaft 82 includes multiple plates84 extending radially outward from the output shaft 82 surface. Themultiple plates 84 include a first set of circumferentially extendingplates 84A positioned around the output shaft 82 and a second set oflongitudinally extending plates 84B positioned along the output shaft82. Walls of the plates 84 form generally rectangular pockets, herethirty in number, around the surface of the output shaft 82 into whichthe tangential buckles 70 and axial buckles 60 mount. The plates 84 alsoinclude radial slots. Specifically, longitudinal plates 84B includeradial slots 85B in every rectangular pocket wall around the outputshaft 82 formed by the longitudinal plates 84B for mounting thetangential buckles 70. Similarly, the circumferential plates 84A defineradial slots 85A in every other rectangular pocket wall around theoutput shaft 82 formed by the circumferential plates 84A for mountingthe axial buckles 60. However, the present embodiment only utilizesthree axial buckles displaced within the rectangular pockets in themiddle of the rectangular pocket array. That is, no radial slots 85A arefound on the outer circumferential plates 84A.

Referring again to FIG. 2, as discussed above, a vacuum chamber 86 isintegrally mounted to the output shaft 82 and encloses the rotorassembly 50. The vacuum chamber 86 also encloses the circumferentialplates 84A and longitudinal plates 84B, and is sized to allow the rotorbody 58 and rotor coils 52 to be mounted to the output shaft 82. Theoutput shaft 82 extends beyond the vacuum chamber 86 and the plates 84at both ends. On one end, the output shaft 82 extends to connect to anexternal load that the motor 10 will drive. At the other end, the outputshaft 82 connects to a rotating half of a brushless exciter 16.

The brushless exciter, shown in FIG. 2, includes a rotating disk 16spaced from a stationary disk 14 (e.g., spaced 1-4 mm). Rotating disk 16is formed of a high permeability laminated material (e.g., iron) andincludes a pair of concentric grooves within which a pair of coilwindings is disposed. Stationary disk 14 is similarly formed of a highpermeability material and includes a pair of concentric grooves withinwhich a pair of coil windings is disposed. In essence, this arrangementprovides a transformer having a primary, which rotates relative to asecondary of the transformer (or vice versa). An important feature ofthis particular arrangement is that the flux linkage generated bystationary disk 14 and rotating disk 16 when stationary is the same aswhen the rotating disk rotates. This feature advantageously allowssuperconducting rotor coils 52 to be charged prior to rotating disk 16rotating (i.e., before motor 10 operates). The structure and operationof the brushless exciter is described in U.S. patent application Ser.No. 09/480,430, entitled “Exciter and Electronic Regulator for RotatingMachinery,” filed on Jan. 11, 2000, assigned to American SuperconductorCorporation, assignee of the present invention, and incorporated hereinby reference.

The rotor assembly includes an electromagnetic shield 88 wrapped aroundthe vacuum chamber 86, formed preferably from a non-magnetic material(e.g., aluminum, copper). In embodiments in which vacuum chamber 86 isformed of a different material, such as stainless steel, electromagneticshield 88 can be mechanically located around the outer wall of thevacuum chamber 86. Electromagnetic shield 88 also acts as an inductionstructure (i.e., supports induction currents) and is, therefore,multi-purposed. Specifically, electromagnetic shield 88 intercepts ACmagnetic fields from the stator before they impact the superconductingwindings 26 of the rotor assembly 12. Further, because electromagneticshield 60 acts as an induction structure, it can be used to operate thesynchronous superconducting motor 10 at start-up in an induction mode.The electromagnetic shield 88 allows the superconducting motor 10 tooperate as an induction motor for start up or in a continuous mode as abackup mode in case of a catastrophic failure of the cryogenic systems.This mode of operating a synchronous motor is described in U.S. patentapplication Ser. No. 09/371,692, assigned to American SuperconductorCorporation, assignee of the present invention, and is incorporatedherein by reference.

Referring to FIG. 9, the rotor assembly 50 further includessuperconducting rotor coils 52 mounted to a stainless steel rotor body58 for support. The rotor body 58 also carries the closed cryogeniccooling loop 118 that cools the rotor coils 52. The rotor body 58 istubular with an inner surface 90 and an outer surface 92. The outersurface 92 may be generally cylindrical in shape, or may have flatsmachined to accept the rotor coils 52. The machined flats may, forexample, give the outer surface 92 a general pentagonal, hexagonal orheptagonal shape. In the present invention, twelve flats have beenmachined to accept twelve flat rotor coils 52.

The rotor body 58 includes rotor body ribs 59 to mount the tangentialbuckles 70 and axial buckles 60, which interface with the output shaft82. The rotor body ribs 59 are circumferentially fixed on the innersurface 90 and extend radially inward from the inner surface 90 of therotor body 58.

In this embodiment, the superconductor in the rotor coils 52 is a hightemperature copper oxide ceramic superconducting material, such asBi₂Sr₂Ca₂Cu₃O_(x) or (BiPb)₂, commonly designated BSCCO 2223 or BSCCO(2.1)223. Other high temperature superconductors including YBCO (orsuperconductors where a rare earth element is substituted for theyttrium), TBCCO (i.e., thallium-barium-calcium-copper-oxide family), andHgBCCO (i.e., mercury-barium-calcium-copper-oxide family) are alsowithin the scope of the invention. Rotor coils 52 may be formed withpancake coils either single or double layers. In certain embodiments,double pancake coils with the two coils of a pair being wound from thesame continuous length of superconducting tape may be used. In thiscase, a pancake coil may include a diameter smaller than its associatedpancake coil of the double pancake. An approach for using this approachis described in U.S. Pat. No. 5,581,220, which is assigned to AmericanSuperconductor, the assignee of the present invention, and incorporatedherein by reference. Preferred embodiments are based on the magnetic andthermal properties of high temperature superconducting composites,preferably including superconducting ceramic oxides and most preferablythose of the copper oxide family. The structure and operation of thesuperconducting windings is described in U.S. patent application Ser.No. 09/415,626, entitled “Superconducting Rotating Machine,” filed onOct. 12, 1999, assigned to American Superconductor Corporation, assigneeof the present invention, and incorporated herein by reference.

Referring to FIG. 10, the rotor coils 52, as described above, arefabricated with an internal support 54 to help stabilize the structurebecause the racetrack configuration produces tremendous bending stressesthat attempt to push the superconducting coil assembly apart. Toovercome this limitation, the rotor coils 52 are fabricated in alaminated configuration with internal coil supports 54, alternatingbetween superconducting windings 126 and internal support 54. Externalsupports, such as the inner spacer 140 and the outer spacer 142, do notsufficiently alleviate the internal stresses associated withnon-circular and non-linear configurations, such as the racetrackconfiguration. The addition of internal coil supports 54 combined withthe inner spacer 140 and outer spacer 142 gives mechanical strength tothe rotor coil 52 and reduces the internal strains in thesuperconducting coils 126. The internal strains are reduced by using theinternal coil supports 54 partly because the peak strains are located atthe inside diameter of the superconducting coils 126, far removed fromany external support structures that could be employed.

In the present embodiment, the internal coil support 54 is 40-mil thickstainless steel. However, it can be appreciated that various thicknessesand materials (such as copper or fiberglass composites) would work fortheir intended purposes, as various embodiments would require differentthicknesses to optimize performance. In certain embodiments, a thermallyconductive coating can be applied to the internal coil support 54 toprovide better heat conductivity to cryogenic cooling tubes 118 locatedwithin the rotor body 58. For example, the internal coil support can becoated with copper.

A fastener can be used to tie the internal coil supports 54 together.For example, the layers can be mechanically fastened together by passinga bolt, or multiple bolts, through the internal coil supports 54 at apoint within the annular opening 136 created by the superconductorwindings 126 and fixing the assembly and top cap 144 to the rotor body58. The bolts tie the internal coil supports 54 together into a unitarywhole, resulting in even greater mechanical strength. The rotor coils 52can also be epoxied together, with or without fasteners, to further fixthe lamination together.

The internal coil support member 54 will also have various openings (notshown) to facilitate electrical connections between adjacentsuperconductor windings. Each superconducting coil assembly in the rotorcoils 52 has to be electrically connected. Since the internal supportmembers 54 are placed between each rotor coil 52, an opening must beprovided to allow the electrical connection between each rotor coil 52.

Referring to FIGS. 11 and 13B, the axial buckles 60 are assembled in therotor assembly 50 to prevent axial movement between the rotor body 58and the output shaft 82. The axial buckles 60 also thermally isolate thecryogenically cooled rotor body 58 from the output shaft 82 by using athermally isolating coupling band 66 between the coupling members 62 and64.

A generally U-shaped coupling member 62 is mounted to the rotor body 58by sliding the open end over the rotor body rib 59. The rotor body rib59 constrains the U-shaped coupling member 62 in the axial direction.Two smaller coupling members 64 are mounted in opposing radial slots 85Ain the circumferential output shaft plates 84A by a narrow shoulder 65on one face of the smaller coupling members 64. The narrow shoulder 65slides into the radial slot 85A while the rest of the smaller couplingmember 64 is wider than the radial slot 85A, thereby preventing thesmaller coupling member 64 from moving beyond the slot 85A. The twosmaller coupling members 64 are mechanically coupled to the U-shapedcoupling member 62 by thermally isolating coupling bands 66. Thethermally isolating coupling bands 66 are Para-aramid/Epoxy straps. Byusing thermally isolating coupling bands 66, the output shaft 82 and therotor body 58 are thermally isolated from each other since the couplingbands 66 are the only direct connection between the U-shaped couplingmember 62 and the smaller coupling members 64. This thermal isolationhelps prevent the output shaft 82 from acting as a heat sink.

The coupling bands 66 wrap around spherical ball end couplings 69mounted in the U-shaped coupling member 62 and the smaller couplingmembers 64. The spherical ball end coupling 69 in one of the smallercoupling members is a cam 68, which is used to preload the couplingbands 66. Surrounding the cylindrical pins 72 and cam 68 are sphericalball ends 69. The spherical ball end couplings 69 hold the coupling band66 and provide misalignment take-up. The spherical ball end couplings 69maintain even loading to the coupling band 66. The coupling bands 66 arepreloaded by turning the cam 68 to vary the tension. The coupling bands66 are 180° apart, which allows one cam to tension both coupling bands66 at the same time and put both coupling bands 66 in uniaxial tension.This configuration also constrains the rotor body 58 and output shaft 82in both axial directions. The adjustability of the cam 68 allows eachaxial buckle 60 to be quickly preloaded by adjusting to anymanufacturing tolerance differentiation within the coupling bands 66,thereby facilitating a quicker build time for the rotor assembly 50.

Referring to FIGS. 12 and 13A, the tangential buckles 70 are assembledin the rotor assembly 50 to transfer the rotational forces between therotor body 58 and the output shaft 82. The tangential buckles 70 alsothermally isolate the cryogenically cooled rotor body 58 from the outputshaft 82 by using a thermally isolating coupling band 66 between thecoupling members 72 and 74.

An X-shaped coupling member 74 is mounted to the output shaft 82 by tworecessed slide mounting areas 78 located on opposing legs of theX-shaped coupling member 74. These recessed slide mount areas 78 arepositioned such that the X-shape coupling member 74 mounts parallel tothe axis of the output shaft 82. The recessed slide mounting areas 78slide down into the radial slot 85B in the longitudinal plates 84B,which constrain the X-shaped coupling 74 in the circumferential andaxial directions. Two spherical ball end coupling 69 are mounted betweenthe rotor body ribs 59 by pressing a cylindrical pin 72 through therotor body ribs 59 and a spherical ball end coupling 69. The sphericalball end couplings 69 are mechanically coupled to the X-shaped couplingmember 74 by thermally isolating coupling bands 66. As discussed above,the thermally isolating coupling bands are Para-aramid/Epoxy straps,which thermally isolate the rotor body 58 from the output shaft 82.

Referring to FIGS. 12 and 12B, the coupling bands 66 wrap aroundspherical ball end couplings 69 mounted in the X-shaped coupling member74, in the two legs not defining the recessed slide mounting area 78,and around the spherical ball end coupling 69 mounted in the rotor bodyribs 59. The coupling bands 66 are mounted 180° apart, which allows bothcoupling bands to be in uniaxial tension. The X-shaped coupling member74 defines an opening 80 therethrough sized to accept a spring 96, whichpreloads both bands in uniaxial tension. The opening 80 is defined so asto be perpendicular to the axis of the output shaft 82 when the X-shapedcoupling member 74 is mounted to the output shaft 82, allowing thespring 96 to push the X-shaped coupling member 74 radially outward. Thespring 96 allows the tangential buckle 70 to be preloaded by compressingthe spring 96. The spring 96 also allows for some compliance when thetangential buckle 70 is assembled within the rotor assembly 50. Thecompressed spring 96 allows each tangential buckle 70 to be quicklypreloaded by adjusting to any manufacturing tolerance differentiationwithin the coupling bands 66, thereby facilitating a quicker build timefor the rotor assembly 50. The preload feature also facilitates loadingthe coupling bands 66 in pure tension. By loading the coupling bands 66in pure tension, the assembly can transmit an extremely large torquebetween the rotor body 58 and the output shaft 82.

The longitudinal output shaft plates 84B are sized within axial slots(not shown) in the rotor body 58 such that they will bottom out during ahigh fault loading situation, thereby preventing the coupling bands 66from breaking. If a sudden shock load is applied to the motor 10,metal-to-metal contact will occur. The advantage to designing such ashock system is that the coupling bands 66 do not have to be sized forfault and shock loads, which would make the coupling bands 66impractical.

Referring to FIGS. 2, 14 and 15, a cryogenic cooling system 100 is usedto maintain a cryogenic fluid at cryogenic temperatures and move thecryogenic fluid to and from a cryogenic cooling loop 118 locatedadjacent and in thermal communication with the rotor coils 52. Thecryogenic fluid is moved through the cryogenic cooling loop 118 by acryogenically adaptable fan 114. This system helps maintain the rotorcoils 52 at cryogenic temperatures, because the superconducting rotorcoils 52 have to be maintained at cryogenic temperatures (i.e., below−79° C.) to operate properly and efficiently. The cryogenic coolingsystem 100 includes multiple cryogenically cooled surfaces 102, hereGifford-McMahon cold heads, mounted in cryocooler assemblies 104, amounting flange 106 and a cryogenically adaptable fan 114. The cryogeniccooling system 100 utilizes a closed loop system for efficiency and easeof maintenance.

The advantage of more than one cryogenically cooled surface 102 isefficiency and ease of maintenance. First, more than one cryogenicallycooled surface 102 in series will allow each cryogenically cooledsurface 102 to work less to lower the temperature of the cryogenicfluid. Also, if one cryogenically cooled surfaces 102 malfunctions, theredundancy in the system will be able to overcome the loss. Further, ifone cryogenically cooled surface 102 does malfunction, themalfunctioning cryogenically cooled surface 102 can be isolated from thesystem by proper valving, and maintenance performed without shuttingdown the system or introducing contaminants into the system.

The cryocooler assembly 104 mounts to the outside of the superconductingmotor 10 via a mounting flange 106 fixed to the housing 12. The fixedcryocooler assembly 104 is in fluidic communication with a cryogeniccooling loop 118. In an embodiment with a rotating thermal load, such asthe rotor coils 52, the cryocooler assembly 104 interfaces with therotating cryogenic cooling loop 118 by interfacing with a rotary seal108, here a ferrofluidic rotary seal. The rotary seal 108 allows thecryocooler assembly 104 to remain fixed while the cryogenic cooling loop118 rotates with the rotor assembly 50. The cryocooler assembly 104 ismaintained stationary, rather than rotating, due to undesirable highgravity heat transfer seen internal to the cryocooler assembly 104 if itwere to rotate. The cryogenic cooling loop 118 is in thermalcommunication with the rotor coils 52, maintaining the rotor coils 52 ata cryogenic temperature.

The cryocooler assembly 104 is open to the vacuum chamber 86 of therotor assembly 50. Keeping the internal area of the cryocooler assembly104 at vacuum helps to isolate the portion of the cryogenic cooling loop118 that is located within the cryocooler assembly 104 from outsidetemperatures. The vacuum isolation further helps improve the efficiencyof the cryogenically cooled surfaces 102.

The cryogenic fluid, helium in this embodiment, is introduced into thesystem from a cryogenic fluid source 116. The cryogenic cooling systemis a closed system, but cryogenic fluid will have to be addedperiodically should any leaks develop. Other cryogenic fluids, such ashydrogen, neon or oxygen, may also be used.

The cryogenic fluid must be moved from the cryocooler 104 to the portionof the cryogenic cooling loop 118 located within the rotor body 58. Acryogenically adaptable fan 114 is employed to physically move thecryogenic fluid. The advantage of a fan is that a fan does not require aheat exchanger to warm the fluid to the temperature of an ambientcompressor, is inexpensive and is relatively small. In comparison, aprior art room temperature compressor in conjunction with a heatexchanger is more expensive and is much larger. Further details of theoperation of the cryogenic cooling system 100 can be found in U.S.patent application Ser. No. 09/480,396, entitled “Cooling System for HTSMachines,” filed on Jan. 11, 2000, assigned to American SuperconductorCorporation, assignee of the present invention, and incorporated hereinby reference.

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, the components described could be adapted to produce othersuperconducting rotating machines, such as a superconducting generator.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A superconducting coil assembly comprising: aplurality of superconducting windings disposed along a longitudinal axisof the coil assembly, each superconducting winding being non-circular inshape and formed of a strain-sensitive material; internal coil supportmembers positioned between adjacent ones of the plurality ofsuperconducting windings and configured to reduce internal straingenerated in the windings; the internal coil support members beingpositioned between adjacent ones of the plurality of superconductingwindings and on a wide surface of the superconducting windings, therebyforming a laminated stack alternating between the internal supportmembers and adjacent ones of the superconducting windings, at least someof the internal coil support members including a composite material; andepoxy for impregnating the laminated stack.
 2. The superconducting coilassembly of claim 1 wherein the non-circular shape is a racetrack shapedefining a pair of opposing arcuate end sections and a pair of opposingsubstantially straight side sections.
 3. The superconducting coilassembly of claim 1 further comprising: at least one fastener; and arotor body; the rotor body being cylindrical in shape and having anouter surface, the fastener being passed through the superconductingcoil assembly at a point in an annular opening defined by thesuperconducting windings and fastened into the rotor body outer surface,thereby mounting the coil assembly to the rotor body and tying theinternal coil support members together into a unitary whole.
 4. Thesuperconducting coil assembly of claim 3 wherein the fastener is a bolt.5. The superconducting coil assembly of claim 1 wherein the internalcoil support members further comprise openings to allow for electricalconnections between adjacent superconducting windings separated by theinternal coil support member.
 6. The superconducting coil assembly ofclaim 1 wherein the at least one internal coil support is stainlesssteel.
 7. The superconducting coil assembly of claim 1 wherein thestrain-sensitive material is a superconducting ceramic oxide.
 8. Thesuperconducting coil assembly of claim 7 wherein the superconductingceramic oxide is a high temperature superconducting ceramic oxide. 9.The superconducting coil assembly of claim 8 wherein the hightemperature superconducting ceramic oxide is BSCCO
 2223. 10. Thesuperconducting coil assembly of claim 1 wherein the superconductingwindings are single pancake coils.
 11. The superconducting coil assemblyof claim 1 wherein the superconducting windings are double pancakecoils.
 12. The superconducting coil assembly of claim 1 wherein theinternal coil support members are 0.040 inches thick.
 13. Thesuperconducting coil assembly of claim 1 wherein the at least some ofthe internal coil support members are formed of a fiberglass composite.14. The superconducting coil assembly of claim 1 wherein the at leastsome of the internal coil support members include a thermally conductivecoating.
 15. The superconducting coil assembly of claim 1 furthercomprising an end cap positioned at an end region of the plurality ofsuperconducting windings.